Bob, many thanks for that info...
Having examined the case with a magnifying glass I was pretty sure how to get the cover off.
Press the two buttons in and slide towards in the direction of the back.
Inner Battery Cover Which Just Lifts Off
LS14250 Battery Still In Place With Minor Corrosion
I now have a copy of the manual for the non audio model so with luck it is similar
A new battery is also now on order and I will report back after it arrives